Preheat oven to 325 degrees. Prepare a 9X13 pan with nonstick spray.
In a large bowl, combine the cake mix, oil, eggs, buttermilk and butter.
After the batter is well combine, pour half of it into your prepared pan.
In another bowl, combine the brown sugar, nutmef and cinnamon.
Sprinkle on top of the batter in your pan, covering the entire area.
Take the remaining batter and pour it over the brown sugar mixture.
Using a knife, swirl the top layer of batter and brown sugar mixture together to make a pretty swirl design
Bake for 40-50 minutes.
While baking, prepare the icing by mixing the powdered sugar, milk and vanilla together.
When the cake comes out of the oven, pour the icing all over.
Cool for 15-20 minutes before cutting.
Serve & enjoy!